Cleaning electroplating substrate holders using reverse current deplating

ABSTRACT

Provided are cleaning methods and systems to remove unintended metallic deposits from electroplating apparatuses using reverse current deplating techniques. Such cleaning involves positioning a cleaning (deplating) disk in an electroplating cup similar to a regular processed substrate. The front surface of the cleaning disk includes a corrosion resistant conductive material to form electrical connections to deposits on the cup&#39;s surfaces. The disk is sealed in the cup and submerged into a plating solution. A reverse current is then applied to the front conductive surface of the disk to initiate deplating of the deposits. Sealing compression in the cup may change during cleaning to cause different deformation of the lip seal and to form new electrical connections to the deposits. The proposed cleaning may be applied to remove deposits formed during electroplating of alloys, in particular, tin-silver alloys widely used for semiconductor and wafer level packaging.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/618,337, titled “CLEANING ELECTROPLATING SUBSTRATE HOLDERSUSING REVERSE CURRENT DEPLATING,” and filed on Mar. 30, 2012, which isincorporated by reference herein in its entirety.

BACKGROUND

Recent advances in semiconductor fabrication and processing has led toincreased use of electroplating to deposit a variety of materials onsemiconductor devices. Such materials include electroplated copper,nickel, and tin-silver alloys. Electroplating tin-silver alloysoftentimes leads to spurious metal buildup around the lip seal and cupregions of a substrate holder assembly (sometimes implemented as aclamshell assembly). This buildup, referred to as “lip seal plating”and/or “cup bottom plating,” depending on its location, may in somecircumstances cause the seal formed between the substrate and lip sealto fail, resulting in contamination of inner portions of the clamshellassembly with potentially corrosive electroplating solution.

SUMMARY

Certain embodiments herein relate to methods and apparatus for removingunintended metallic deposits from electroplating apparatuses usingreverse current deplating techniques.

In one aspect of the embodiments herein, a method of mitigating anelectroplating deposit on a substrate holder configured to hold a wafersubstrate in an electroplating cell is provided. The method may includeproviding a deplating disk in the substrate holder when theelectroplating cell is not being used for electroplating metal on asubstrate, such that the deplating disk makes electrical contact with aplurality of electrical contacts in the substrate holder, immersing thedeplating disk and substrate holder into an electroplating solutionwithin the electroplating cell, and applying an anodic potential to thedeplating disk under conditions sufficient to remove, at leastpartially, the electroplating deposit from the substrate holder, wherethe deplating disk has a size and a shape matching those of a standardsemiconductor wafer.

In certain embodiments, the deplating disk has metal or othernon-corrosive conductive material at least around its perimeter in aregion for making electrical contact with the plurality of electricalcontacts in the substrate holder. The peripheral metal may be a noblemetal, and in some cases includes platinum, palladium, gold, iridium,osmium, ruthenium, or a combination thereof. In certain embodiments, theperipheral metal may include titanium, tantalum, or chromium. Thedeplating disk may also include an insulator in a central region of thedeplating disk. In other cases, the metal located on the periphery ofthe deplating disk also extends over the entire deplating face of thedeplating disk. In some implementations, the deplating disk has adiameter of about 450 mm, while in other implementations the deplatingdisk may have a diameter of about 300 mm.

The electroplating deposit to be removed may be an alloy in certaincases. In one example, the electroplating deposit to be removed includessilver and tin. The substrate holder may also include a lip seal invarious implementations. The method of mitigating the electroplatingdeposit may also include compressing the deplating disk on the lip sealto cause the deplating disk to come into electrical contact with theelectroplating deposit. In some embodiments, after applying the anodicpotential to the deplating disk, the compression on the deplating diskis varied to cause the lip seal to flex to a new position and therebycause the deplating disk to come into electrical contact with theelectroplating deposit, and a further anodic potential is applied to thedeplating disk under conditions sufficient to remove the electroplatingdeposit from the substrate holder. In certain implementations, themethod also includes moving the deplating disk to a storage area for thedeplating disk, where the storage area is separate from a storage areafor semiconductor wafers.

In another aspect of the embodiments herein, a method of cleaning asemiconductor plating apparatus is provided, the method includingproviding a deplating disk in a wafer holder assembly, where thedeplating disk includes corrosion-resistant conductive material aroundat least the periphery of the deplating disk, and where the wafer holderassembly includes a peripheral lip seal and a plurality of electricalcontacts, where the wafer holder assembly is positioned in anelectroplating cell, and where the wafer holder assembly has metaldeposited thereon, and sealing the deplating disk in the wafer holderassembly by applying a force to the deplating disk to thereby deform thelip seal and provide a liquid-tight seal, and thereby establishelectrical communication between the plurality of electrical contactsand the metal deposited on the wafer holder assembly, submerging thedeplating disk in cleaning solution in the electroplating cell, andapplying cathodic current to an anode in the electroplating cell andapplying anodic current to the deplating disk to thereby remove metaldeposited on the wafer holder assembly.

In practicing this embodiment, the force applied to the deplating diskmay be the same as the force applied to a plating wafer when a platingwafer is sealed in the wafer holder assembly. Conversely, the forceapplied to the deplating disk may be different (less force or moreforce) as compared to a force applied to a plating wafer when a platingwafer is sealed in the wafer holder assembly. The deplating disk mayinclude an insulated center portion, in certain cases.

Often but not always, the metal deposited on the wafer holder assemblyis an alloy. In certain embodiments, the metals that form the alloy mayhave reduction potentials that differ by about 0.1 V or more, and insome cases by about 0.3 V or more. In a particular case, the alloy mayinclude tin and silver. The operation of applying anodic current to thedeplating disk may be carried out in various ways. In one case, thecurrent is applied in a galvanostatic mode, while in another case thecurrent is applied in a potentiostatic mode. Where the potentiostaticmode is used, the anodic current may be applied to the deplating diskuntil a target current density is reached. Where the galvanostatic modeis used, the anodic current may be applied until a target potential isreached. The target potential may be a potential at or slightly beyondthe oxidation potential of a component metal in the metal deposited onthe wafer holder assembly. Further, when practicing the galvanostaticmode, the current density may be between about 0.5-2 Amps per squaredecimeter. In a particular example, applying anodic current to thedeplating disk includes applying current at a first voltage potentialand applying current at a second voltage potential, where the first andsecond voltage potentials correspond to potential that are slightlybeyond the voltage potentials of each of two metals in the alloy. Whileapplying anodic current to the deplating disk, the current density maybe controlled to avoid oxygen generation. In one case, the anodiccurrent may be applied to the deplating disk in pulses.

In some embodiments, the cleaning solution is the same as a solutionused for electroplating a wafer. In a particular example, the cleaningsolution includes between about 0.2-1.5 g/L silver ions, between about30-100 g/L tin ions, and between about 70-350 g/L acid.

The method may also include rotating the deplating disk at 5 RPM orgreater during at least the applying current operation. Further, themethod may include rotating the deplating disk in a first direction andthen in a second direction. In some embodiments, the deplating disk maybe removed from the cleaning solution, then spun between about 100-600RPM to reclaim the cleaning solution from the deplating disk, thenrinsed with deionized water, and then spun to remove the deionizedwater. This procedure helps conserve cleaning solution and preserve thedeplating disk. In some cases, the deplating disk may be automaticallymoved to a deplating disk storage area after cleaning.

In certain implementations, the method may be implemented such that itis repeated periodically. In one example, the method repeats after atime interval. In another example, the method is repeated after acertain number of plating wafers are plated. In a third example, themethod is repeated after a certain amount of charge has passed. In afourth example, the method is repeated after a signal is received, wherethe signal corresponds to a condition where the wafer holder assemblyhas a threshold amount of metal deposited thereon. This last example maybe especially useful where there is some detection mechanism in place todetect when cleaning is appropriate.

In certain embodiments, applying current to the deplating disk maydissolve metal deposited on the wafer holder assembly that is withinabout 10 mm of the deplating disk-lip seal interface. Alternatively orin addition, applying current to the deplating disk may dissolve atleast about 25% of the metal plated on the wafer holder assembly.

In another aspect of the embodiments disclosed herein, a plating toolfor plating semiconductor wafers is provided, including at least oneplating module including an electroplating cell configured to contain ametal-ion-containing electroplating solution; a plating cell waferholder assembly including a peripheral lip seal and a plurality ofelectrical contacts, where the plating cell wafer holder assembly isconfigured to receive a semiconductor wafer, and the peripheral lip sealis configured to form a liquid-tight seal between an edge of thesemiconductor wafer and the peripheral lip seal to thereby preventelectroplating solution from coming into contact with the plurality ofelectrical contacts; a deplating disk including a corrosion resistantconductive material (in some cases a noble metal) around at least theperiphery of the deplating disk, where the noble metal of the deplatingdisk is positioned to be in electrical communication with the pluralityof electrical contacts and in close proximity to the peripheral lip sealsuch that when anodic current is applied to the cleaning disk during acleaning mode, metallic deposits formed on the lip seal and plating cellwafer holder assembly are removed; and a power supply configured toprovide anodic current to an anode and cathodic current to asemiconductor substrate in a plating mode, and cathodic current to theanode and anodic current to the deplating disk in the cleaning mode.

The plating tool may also include a wafer storage area configured tostore the semiconductor wafers when they are not being electroplated inthe plating module. The power supply may be configured, in certaininstances, to supply current according to the plating mode when thesemiconductor substrate is present in the plating module, and to supplycurrent according to the cleaning mode when the deplating disk ispresent in the plating module. The plating tool may also include anauxiliary electrode capable of being in electronic communication withthe semiconductor wafer when at least a portion of the semiconductorwafer is immersed in the electroplating solution.

The deplating disk may include one or more exposedthermodynamically-oxidizable but kinetically passivatedcorrosion-resistant metals. As an example, in certain implementations,the kinetically passivated corrosion resistant metal includes at leastone of the non-noble materials titanium, tantalum, vanadium, tungsten,niobium and/or chromium. Further, the corrosion resistant conductivematerial on the deplating disk may include a “noble” material selectedfrom the group consisting of platinum, palladium, gold, iridium, osmium,ruthenium, rhodium, and combinations thereof. These noble metals aregenerally not passivated, but rather are corrosion resistant due totheir thermodynamic nature (e.g., high oxidation potentials). In certainimplementations, the corrosion resistant conductive material of thedeplating disk has an oxygen formation overvoltage below about 0.5 V. Insome embodiments, the deplating disk includes an insulated centerportion surrounded by a peripheral conductive portion. The width of theconductive portion, in certain cases, is between about 1-10 millimeters,or between about 2-6 millimeters. The peripheral conductive portion mayhelp define a gap between the center insulated portion of the deplatingdisk and the lip seal of the plating tool. In some implementations, thisgap is between about 1-5 millimeters. In some embodiments, the deplatingdisk may include a semiconductor substrate that is at least partiallycoated with a corrosion resistant conductive material. Typically, thedeplating disk will have a deplating face and an opposite face. Incertain implementations, the deplating face includes a step extendingaway from the opposite face of the deplating disk. The step may be atleast 1 mm long, and be positioned between about 0.1 and 5 mm inwardfrom the perimeter of the deplating disk. A gap between the step and lipseal may be about 2 millimeters or less. In some embodiments, the stepphysically contacts the lip seal when engaged.

In an additional aspect of the embodiments herein, a plating tool forplating semiconductor wafers is provided, including at least one platingmodule including an electroplating cell configured to contain ametal-ion-containing electroplating solution; a plating cell waferholder assembly including a peripheral lip seal and a plurality ofelectrical contacts, where the plating cell wafer holder assembly isconfigured to receive a workpiece that is either a semiconductor waferor a deplating disk, where the peripheral lip seal is configured to forma liquid-tight seal between an edge of the workpiece and the peripherallip seal to thereby prevent electroplating solution from coming intocontact with the plurality of electrical contacts; a power supplyconfigured to provide anodic current to an anode and cathodic current tothe workpiece in a plating mode, and cathodic current to the anode andanodic current to the workpiece in a cleaning mode; a deplating-diskstorage holder; a wafer storage area; a robotic arm; and a controllerconfigured to direct the robotic arm to move the deplating disk betweenthe deplating disk storage holder and the plating module, and to movethe semiconductor wafer between the wafer storage area and the platingmodule, and to direct the power supply to supply current according tothe plating mode when the semiconductor wafer is present in the platingmodule, and to supply current according to the cleaning mode when thedeplating disk is present in the plating module.

The plating tool may also include at least one workpiece rinsing module,and/or an auxiliary electrode capable of being in electroniccommunication with the work piece when at least a portion of theworkpiece is immersed in the electroplating solution. Further, the lipseal and/or plating cell wafer holder assembly may have a metallic alloydeposited thereon. The metals which form the alloy may have reductionpotentials that differ by over about 0.1 V, and in some cases by overabout 0.3 V. In certain examples the alloy includes tin, and in oneparticular example the alloy includes tin and silver. The plating toolmay also include a cone that sits atop the workpiece. Separately or inaddition, the plating tool may include a force applicator that applies avariable force to the workpiece, either directly or indirectly. In manycases the lip seal is compressible. This compressibility allows the lipseal to deform, thereby providing additional electrical connections tometallic deposits.

In another aspect of the embodiments herein, a deplating disk isprovided. The deplating disk serves as an anode when used to mitigate anelectroplating deposit on a substrate holder that is configured to holda substrate in an electroplating cell while applying a cathodic currentto electroplate metal on the substrate. The deplating disk may include aperimeter region including a corrosion-resistant conductive materialpositioned for making electrical contact with a plurality of electricalcontacts in the substrate holder, and an interior region including aninsulator, where the deplating disk is shaped as a circular wafer havinga diameter or about 300 or about 450 mm. In certain implementations, thedeplating disk has a different diameter corresponding to the diameter ofa substrate that is plated in the plating tool in which the deplatingdisk is used. While typical substrates are 300 or 450 mm, any sizedeplating disk may be used, so long as it is close in diameter to thetype of substrate that is plated in the particular apparatus. Oftentimes, the corrosion resistant conductive material is metal. In certainimplementations the metal may be a noble metal.

These and other features will be described below with reference to theassociated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a wafer resting on a lip seal in a plating apparatus, withno unwanted metal deposits present.

FIG. 1B-1D show a wafer resting on a lip seal in a plating apparatuswith an unwanted metal deposit present.

FIG. 2 shows a proposed mechanism for the deposition of tin-silvermaterial on portions of a plating apparatus.

FIG. 3 is a flowchart for a method of cleaning an electroplatingapparatus according to a disclosed embodiment.

FIG. 4 illustrates the cleaning disk/lip seal interface as greateramounts of downward force are applied to the cleaning disk.

FIGS. 5A and 5B show top-down and side views of a cleaning disk,respectively.

FIG. 6 shows an embodiment of a cleaning disk having a step and engagingwith the lip seal.

FIG. 7 shows a cross-sectional schematic of an electroplating cellaccording to a disclosed embodiment.

FIG. 8 demonstrates an electroplating apparatus that may be used inaccordance with the embodiments herein.

FIG. 9 shows a multi-station process tool according to an embodimentdisclosed herein.

DETAILED DESCRIPTION

In this application, the terms “semiconductor wafer,” “wafer,”“substrate,” “wafer substrate,” and “partially fabricated integratedcircuit” are used interchangeably. One of ordinary skill in the artwould understand that the term “partially fabricated integrated circuit”can refer to a silicon wafer during any of many stages of integratedcircuit fabrication thereon. Further, the terms “electrolyte,” “platingbath,” “bath,” and “plating solution” are used interchangeably. Thefollowing detailed description assumes the invention is implemented on awafer. However, the invention is not so limited. The work piece may beof various shapes, sizes, and materials. In addition to semiconductorwafers, other work pieces that may take advantage of this inventioninclude various articles such as printed circuit boards and the like.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the presented embodiments.The disclosed embodiments may be practiced without some or all of thesespecific details. In other instances, well-known process operations havenot been described in detail to not unnecessarily obscure the disclosedembodiments. While the disclosed embodiments will be described inconjunction with the specific embodiments, it will be understood that itis not intended to limit the disclosed embodiments.

Integrated Circuit Fabrication, Electroplating Process and ContaminationSources

Electrochemical deposition may be employed at various points in theintegrated circuit fabrication and packaging processes. At theintegrated circuit (IC) chip level, damascene features are created byelectrodepositing copper within vias and trenches to form multipleinterconnected metallization layers. Above the multiple metallizationlayers, the packaging of the chip occurs. Various wafer level packaging(WLP) structures may be employed, some of which may contain alloys orother combinations of two or more metals or other components. Forexample, the packaging may include one or more bumps made from solder orrelated materials. A typical example of a plated bump starts with aconductive substrate seed layer (e.g. a copper seed layer) having anunder bump diffusion layer of plated nickel (e.g., between about 1-2micrometers thick and 100 micrometers wide) under a layer containinglead tin solder plated pillars (e.g., 50 to 100 micrometers thick and100 micrometers wide). After plating, photoresist stripping, and etchingof the conductive substrate copper seed layer, the pillar of solder iscarefully melted or reflowed to create a solder bump or ball attached tothe under bump metal.

In another scheme, which may be referred to as a copper pillar and micropillar scheme, an under bump of a non-solder plated pillar metal such ascopper, nickel, or a combination of these two, is created below a solderfilm. In this scheme, the solder film is typically much thinner andsmaller than in the scheme above. In the copper pillar and micro pillarapproaches, it is beneficial to have tight/precise control of featurepitch and separation. The copper pillars may be, for example, about 50microns or less in width, while the features are separated from oneanother by only about 75-100 micrometers center to center. The copperstructures may be about 20-40 micrometers in height. On top of thecopper pillar, a nickel barrier film, e.g., about 1-2 micrometers thick,may be used to separate the copper from the tin-containing solder and,thereby, to avoid a solid state reaction to form various mechanicallyand chemically undesirable bronzes. Finally, a solder layer, typically20-40 micrometers in thickness is added. This scheme also enables areduced amount of solder for the same features size, leading to a lowercost and a lower total amount of lead (in lead-containing solders) inthe chip.

Recently, a move away from lead-containing solders has increased inmomentum due to environmental and health-safety concerns. Tin-silversolder alloy bumps are of particular interest. Lead-tin materialsprovide good quality “bumps” for packaging and are very easy to plate.However, due to the high toxicity of lead, lead-containing solders arebeing used less frequently. For example, the RoHS initiative (Directive2002/95/EC of The European Parliament) requires entities to change fromthe established tin-lead process to a lead free process. Replacementbump materials may include indium, tin, tin-silver binary materials,tin-bismuth binary materials, and tin-silver-copper ternary materials.Without going into too many details, pure tin can suffer from a numberof fundamental limitations and application difficulties, such as thetendency to form large single grained balls with varying crystalorientations and thermal expansion coefficients, and “tin whiskers”which can lead to interconnect-to-interconnect shorting. The binary andternary materials generally perform better and alleviate some of thesepure tin issues, at least in part by precipitating a large number ofsmall grain inclusions of the non-tin component as part of the soldermelt-to-solid state freezing process. Included in such compositions aresilver-tin alloys.

Deposition of silver-tin alloys is accomplished by a process thatfrequently employs an inert anode (rather than the potentially moredesirable “active” or soluble anode). Part of the difficulty in using anactive anode for this and similar systems results from the very widelyseparated electrochemical deposition potentials of silver and tin; thestandard electrochemical potentials (Eos) of the metals are separated bymore than 0.9 volts (Ag⁺/Ag: 0.8V NHE, Sn⁺²/Sn: −0.15V). Becauseelemental silver is substantially more noble and inert than elementaltin, the silver will undergo a displacement reaction and electroplateout of solution onto the surface of a tin anode or tin/silver anode.This chemical “short circuit” removes (strips or extracts) therelatively low concentration of silver from the plating solutioncontinuously, resulting in both an uncontrollable process as well as theformation of reduced silver metal on the tin anode.

Methods and apparatus for efficient and high-quality plating when usingpotential-differing set of metals are described in the following U.S.patent applications: U.S. Provisional Patent Application No. 61/502,590,filed Jun. 29, 2011, entitled “ELECTRODEPOSITION WITH ISOLATED CATHODEAND REGENERATED ELECTROLYTE”; U.S. Provisional Patent Application No.61/418,781, filed Dec. 1, 2010, entitled, “ELECTROPLATING APPARATUS ANDPROCESS FOR WAFER LEVEL PACKAGING”; and U.S. patent application Ser. No.13/172,642, filed Jun. 29, 2011, entitled, “CONTROL OF ELECTROLYTEHYDRODYNAMICS FOR EFFICIENT MASS TRANSFER DURING ELECTROPLATING.” Eachof these applications is incorporated by reference herein in itsentirety.

Despite the existence of high quality plating regimes when plating twoor more metals, where there is a large difference in the platingpotentials, there remains the issue of unwanted metal plating onsurfaces of the electroplating cup and how to address these unintendeddeposits. Although the discussion herein is couched in terms oftin-silver plating, it applies equally to any unwanted deposits.

Described herein are methods and apparatus for automated cleaning ofsubstrate handling apparatus, particularly apparatus that hold asemiconductor substrate and rotate the wafer during plating or othersemiconductor fabrication process. This processing often leaves unwanteddeposits on the substrate handling components, such as electroplatingcups, which may be also referred to as clamshells because of theirconstructions. As used herein, the term electroplating deposit refers tomaterial which has been electroplated onto a surface. This material istypically metal, and in many cases it is a metallic alloy. In practicingthe embodiments herein, the electroplating deposit is typically found ona surface of an electroplating apparatus. Particularly problematic fromthe contamination perspective is the area about the circumference of thesubstrate, e.g., where the substrate is sealed by a lip seal of theelectroplating cup.

While the discussion herein focuses on substrate holders constructedwith a cup and lip seal, other arrangements may be used. As used herein,the term substrate holder refers to any part of an electroplatingapparatus which supports a substrate during plating. A substrate holdergenerally has some portion that is exposed to electrolyte duringelectroplating. The terms substrate holder, wafer holder, work pieceholder, electroplating cup, plating cup, cup, clamshell holder, andclamshell are used interchangeably. A variety of wafer holders can beused in practicing the embodiments herein. The term lip seal generallyrefers to a portion of the wafer holder that engages with the edge ofthe wafer and creates a seal that protects the interior of the waferholder (including the electrical contacts for connecting to the wafer)from electroplating solution while exposing the entire plating face ofthe wafer to electroplating solution. Any of a variety of lip sealdesigns may be used. In certain cases the lip seal is not part of thewafer holder itself, but instead contacts the wafer holder when engaged.The lip seal is often made from an elastomeric material.

For simplicity and clarity, most of the examples herein concernwafer-face-down, “fountain” plating apparatuses. In such apparatuses,the work piece to be plated (typically a semiconductor wafer or othersubstrate) generally has a substantially horizontal orientation (whichmay in some cases vary by a few degrees from true horizontal for somepart of, or during the entire immersion and/or plating process) and maybe powered to rotate during plating, yielding a generally verticallyupward electrolyte convection pattern. Integration of the impinging flowmass from the center to the edge of the wafer, as well as the inherenthigher angular velocity of a rotating wafer at its edge relative to itscenter creates a radially increasing sheering flow pattern (parallel tothe face of the wafer). Clamshell-type electroplating apparatuses havinga cup and cone arrangement are frequently used to hold wafers in placebefore and during electroplating. Examples of clamshell and fountainplating cells/apparatuses are included in the Sabre® family ofElectroplating Systems produced by and available from Lam Research, Inc.of Fremont, Calif. Additionally, clamshell fountain electroplatingsystems are described in, e.g., U.S. Pat. No. 6,800,187 filed Aug. 10,2001 and U.S. Pat. No. 8,308,931 filed Feb. 11, 2010, which areincorporated herein by reference in their entireties. While thedescription herein focuses primarily on an orientation with the waferand the holder face down, parallel to the plane of the local earth'ssurface, it is understood that other orientations, such as angled ornormal to the earth surface are not excluded and are also envisioned.

Electroplating of various alloys may cause unintended deposition of oneor more materials forming these alloys on surfaces of substrate holdingapparatuses, such as electroplating cups. FIG. 1A shows a close-upcross-sectional view of part of a wafer holder assembly holding a wafer104. The wafer 104 engages with lip seal 102 during plating, the lipseal operating to provide a fluid-tight seal to prevent plating fluidfrom flowing past this boundary. In FIG. 1A there are no unwanted metaldeposits on the lip seal 102. In FIG. 1B, an unwanted metal deposit 106is present. One root of the presence of the unwanted metal deposit isthe difference in electrochemical potentials of materials forming themetal alloy. Some examples of electroplated alloys that have substantialdifferences in electrochemical reductions potentials include tin-silver,tin-bismuth, tin-copper, tin-silver-copper, tin-indium, lead-indium,nickel-palladium, indium-nickel, nickel-tin and iron-cobalt. Often, thisunintended deposition initiates at an interface between the substrateand apparatus or, more specifically, at an interface between thesubstrate and lip seal. In some cases the deposit may grow, progressingalong the entire surface(s) of the substrate holder, e.g., anelectroplating cup/clamshell.

Without being restricted to any particular theory, a proposed depositionmechanism will now be explained with reference to electroplating oftin-silver alloys. FIG. 1B illustrates tin-silver deposition 106 formedon a working (front) surface of the substrate physically contacting alip seal. This deposition may be caused by a localized absence ofphotoresist due to photoresist edge defects, substrate misalignment, andother causes. Furthermore, a tin-silver layer may be thicker than thephotoresist layer and, as a result, may cause “bump-out” plating.

Some of these tin-silver deposits may break loose from the substrate andremain connected to the lip seal or some other component of theelectroplating cup. This may cause some residual and discontinuoustin-silver deposits to be retained in the lip seal area during wafer(substrate) removal, as shown in FIG. 1C. These deposits can laterbecome potential active nuclei for deposition.

During multiple electroplating cycles, some of these nuclei may becomeconnected and form a semi-continuous conductive surface. When thissurface contacts a portion of the substrate being plated, the conductivesurface also receives some plating material and may work effectively asa seed layer. The initially discontinuous nuclei, which may primarilyinclude tin, can undergo a displacement reaction with silver ions in theelectrolyte (two atoms of silver are deposited for every one atom of tinconsumed), thus yielding a large net volume change. The reactionpromotes the volumetric growth of these nuclei, often causing the nucleito physically connect over time. This reaction is believed to cause anet volume change of about 40%.

Without being limited to a particular theory, it is believed thatspurious deposition of tin-silver alloy occurs as a result, at least inpart, of the significantly differing reduction potentials of atomic tinversus atomic silver, and furthermore, that growth of spurious depositsoccurs via a displacement reaction (Sn+2Ag⁺→Sn²⁺+2Ag) on the surface ofthe lip seal and cup surfaces resulting in a substitution of 2 silveratoms (having oxidation state +1) for every atom of tin (havingoxidation state +2). Once again, without being limited to a particulartheory, it is believed that other metal or alloy deposits formed frommetals having significantly different reduction potentials (e.g., wherethe difference in reduction potentials is greater than 0.1 V, or greaterthan 0.3 V) may lead to the same or similar problems involving spuriousmetallic deposition on the lip seal and cup of an electroplatingassembly.

Overall, the initially discontinuous nuclei turn into a significantplating surface, as shown in FIG. 1D. Some nuclei may also form on otherparts of the cup 108, such as below the lip seal on a protrusion of thecup 108 that supports the lip seal. These on-cup deposits are shown inFIG. 1D as element 106A. Likewise, deposits initially formed on the lipseal may extend into other areas. Other types of deposits and depositionmechanisms may occur and have been observed, such as forming silverand/or tin metal containing heterogeneous colloids.

Oftentimes, unintended deposition can be found away from the wafer-lipseal interface. This evidence points to a process where some edgematerial, rather than being plated and remaining on the substrate, iselectrochemically removed from the substrate and plated on the cupholding apparatus. Without being restricted to any particular theory, itis believed that a deposition rate of this unintended build-up on thelip seal and side walls of the electroplating cup is not linear. Thisspecific deposition rate usually increases towards the later stages ofthe overall electroplating process. A proposed mechanism of tin-silverredistribution is illustrated in FIG. 2. In particular, in one proposal,it is purported that the reaction 2L+2e⁻→2L*⁻ may occur on the waferitself during the plating of silver on bumps and the like, therebycreating a reduced form of the silver ligand (complexing agent) L. It isfurther proposed that the reduced ligand is free to flow and act as areducing agent to react at a nearby surface, and heterogeneously producetwo electrons according to the reaction 2L*⁻→2L+2e⁻, which then reactwith tin ions to form solid tin on the wafer holder apparatus, accordingto the reaction Sn²⁺+2e⁻→Sn.

It is also believed that this unintended deposition essentially acts asa “cathode current thief.” The deposition redirects the currentdistribution in the electroplating cup, in particular near the interfacebetween the lip seal and substrate, leading to degradation of platingnear the substrate edges. The unwanted deposition may also causenon-uniformity in deposition thicknesses and alloy compositions. Oncethe unintended deposition has a substantial coverage, the thievingcurrent becomes large enough to cause severe degradation and defects inpackaging and WLP applications. As such, this unintended deposition maystochastically transition from being relatively insignificant to causingcatastrophic failures, usually in a sudden manner. Examples of thiscatastrophic failure are further illustrated in U.S. Provisional PatentApplication Ser. No. 61/618,337, titled “CLEANING ELECTROPLATINGSUBSTRATE HOLDERS USING REVERSE CURRENT DEPLATING,” and filed on Mar.30, 2012, which was previously incorporated by reference herein in itsentirety.

Because tin-silver alloys (and other like alloys) are often depositedtoward the end of semiconductor manufacturing processes (for example, astin-silver solder contacts), the wafers used in these processes aretypically very expensive, having been through many processing operationsbefore they reach this point in the overall production process.Therefore, the loss or low yield of these wafers due to non-uniformitiescaused by the buildup of metal depositions on the lip seal can beespecially costly.

The undesirable deposition needs to be periodically removed to avoidcatastrophic failures. One way of removing this deposition is to performphysical (e.g., manual) wipe downs using a nitric acid solution. Whendepositions are substantial, some components of an electroplatingapparatus may need to be removed and replaced. Often, these preventiveoperations need to be performed on a daily basis and are challenging inproduction environments with high volumes and multiple tools.

Problems associated with spurious metal deposits on lip seals and otherplating apparatus parts are exacerbated as wafers are designed with moreand more features near the edge of wafers, where they are likely to beimpacted by such plating. Further, the presence of near edge featurespromotes generation of spurious deposits on wafer holders and seals. Theinterference between near-edge features and the lip seal plating is muchgreater when the density of features near the edge is high. The drivetowards more near-edge features results from a desire to maximize thenumber of semiconductor devices that can be obtained from a singlewafer.

While the discussion and examples herein generally focus on the removalof tin/silver buildup, the embodiments may be practiced to remove othertypes of plated metals, as well. The mechanism for the tin/silver buildup process is described in some detail above and elsewhere. Theembodiments herein may also be practiced to address the buildup ofcopper on a plating cup used to deposit copper, such copper depositsbeing formed by the transition of partially reduced cupric ions tocuprous ions, or by the formation of organic-additive reducing speciesat the wafer surface which are swept to the wafer edge, lip seal and cupbottom region.

Wafer Surface:Cu⁺² +e ⁻(wafer surface)→Cu⁺¹, then

Lip seal/Cup Bottom:2Cu⁺¹→Cu+Cu⁺²

or

Wafer Surface:A+e−→A ⁻(formation of reductive additive)

Lip seal/Cup Bottom:Cu⁺²+2A−→Cu+2A

The embodiments herein use a wafer-shaped cleaning disk (i.e., deplatingdisk) made of a chemically inert but conductive medium. As used herein,the terms cleaning disk and deplating disk refer to a disk that isplaced into a substrate holder during a cleaning operation, in the spacewhere a wafer would be placed during plating, and which receives ananodic current during the cleaning operation. A cleaning disk istypically the same shape and size as a normal plating wafer (i.e., thephysical dimensions of the cleaning disk are each within about 5%, orwithin about 1%, of the dimensions of a plating wafer).

During cleaning, the cleaning disk is placed in the plating apparatus inthe space where a wafer is placed during plating, i.e., in the waferholder/cup/clamshell. A reverse current (reverse relative to the currentapplied during a conventional plating process) is applied to dissolvethe unwanted metal deposits. Further details of the methods andapparatus for performing the methods are discussed in the relevantsections below.

Methods

Provided are cleaning methods and systems to remove unintended metallicdeposits from electroplating apparatuses using reverse current deplatingtechniques. An electroplating cup may have deposits formed on at leastthe lip seal that need to be removed during cleaning Deposits may bealso present on other surfaces of the electroplating cup. The methodsherein may be used to remove deposits from the lip seal-wafer interfaceand other surfaces of the electroplating cup. The removal may involvedeplating unintended deposits from one area and, in certain embodiments,may involve partial re-plating of these deposits in other areas,depending on polarization of the solution.

FIG. 3 illustrates a flowchart of a method according to an embodimentdisclosed herein. The method 300 of cleaning an electroplating cup toremove unintended deposits formed on at least a lip seal of theelectroplating cup involves providing a cleaning disk (also referred toas a deplating disk) into the electroplating cup at block 302. Thecleaning disk may have a front surface including a corrosion resistantconductive material (e.g., platinum, palladium, gold, iridium, osmiumruthenium, and combinations thereof). The cleaning disk is generally thesame size as processed substrates. In certain cases, the cleaning diskis formed from a substrate by depositing a layer of corrosion resistantconductive material on one or more of its surfaces, thus allowing aconventional silicon wafer to be converted into a cleaning disk.

Next, the cleaning disk is sealed in the electroplating cup bycompressing the lip seal of the electroplating cup against the frontsurface of the cleaning disk at block 304. During sealing, an edgeportion of the front surface establishes electrical contact with thecontact fingers of the electroplating cup. Also during the sealingoperation, an exposed portion of the front surface establisheselectrical contact with the deposits formed on at least the lip seal. Insome implementations, the cleaning disk is sealed in substantially thesame way as a regular substrate. For example, the level of force usedand the extent of lip seal deformation achieved may be the same (e.g.,within about 5%) during sealing of a cleaning disk as during sealing ofa plating wafer. In other embodiments, the sealing may involve applyingdifferent forces to the cleaning disk as compared to when a regularsubstrate is sealed. This difference in forces may be used to achievedifferent deformation of the lip seal during cleaning and to achievecontact between unintended deposits and the conductive surface of thecleaning disk. In certain embodiments, the force applied to the cleaningdisk may change during the cleaning method while the cleaning diskremains sealed in the electroplating cup. Such changes in force may beused to change the deformation of the lip seal and contact levels toensure more thorough cleaning.

The method then proceeds at block 306 with submerging the exposedportion of the front surface of the cleaning disk into an electroplatingsolution and applying a reverse current to the front surface of thecleaning disk at block 308 using the contact fingers of theelectroplating cup to electrochemically dissolve the deposits in theplating solution. Generally, at the later stages of the removal process,as the amount of metal remaining on the lip seal and cup bottomdecreases, the potential of the disk increases, and oxygen evolutioneventually starts to occur. By monitoring the progression of the voltagevs. time (galvanostatic), or decrease in current vs. time(potentiostatic operations) one can generally detect the completion ofthe removal process.

The cleaning method involves positioning a cleaning (deplating) disk inan electroplating cup similar to a regular processed substrate. Thefront surface of the cleaning disk includes a corrosion resistantconductive material to form electronic connections to deposits on thecup's surfaces. As used herein, corrosion resistant material means amaterial that is resistant to dissolution into the plating electrolyteat any applied potential. For example, the cleaning disk may be awafer-shaped disk made from titanium, tantalum, and/or chromium. Thesematerials, though thermodynamically unstable with respect to water andoxygen, will form a surface layer that is electronically and chemicallyresistive (e.g., a passivated oxide or the like), and therefore do notsubstantially corrode spontaneously or when anodically polarized.Instead, when an anodic current is passed, rather than metalliccorrosion, oxygen evolution will generally be the preferred electrolyticprocess on the corrosion-resistant surface, as long as a sufficientlylarge oxidation potential is applied.

Further, the cleaning disk may be a silicon or other substrate that iscoated with a noble metal such as platinum, palladium, gold, iridium,osmium, and/or ruthenium. As used herein, noble metals are metals of thegroup VIIIB, period 5 and 6 elements, or alloys formed from theseelements, which are resistant to corrosion and oxidation in moist airand are substantially free of a chemically and electronically resistivepassivating film. The applied potential for use of a noble metal isgenerally less than for a passivated metal. The passivated or noblemetal layer may only be present or exposed at the disk edges, and may beelectrically masked everywhere else, such as in the center portion ofthe disk. Coatings with low oxygen formation overvoltage (e.g., 0.4V),such as coatings including noble metals, are preferred from an energyand power consumption prospective.

The disk is sealed in the cup and submerged into a plating solution.During the sealing operation, an edge portion of the front surfaceestablishes an electrical contact with contact fingers of theelectroplating cup. The contact fingers are provided on a sealed side ofthe lip seal. Establishing contact between the front surface of thecleaning disk and the contact fingers may be similar to establishingcontact between a regular substrate and the contact fingers. Sealingcompression in the cup may change during cleaning to cause differentdeformation of the lip seal and to form new electrical connections tothe deposits.

In certain embodiments, the method may involve changing a force appliedto the cleaning disk to seal the cleaning disk against the lip seal.This change in force may be performed while applying the reverse currentto the cleaning disk. The force is changed to achieve differentdeformation of the lip seal. For example, the force may be increased toachieve greater deformation of the lip seal and establish additionalelectrical contacts between the deposits and the exposed portion of thefront surface of the cleaning disk.

FIG. 4 illustrates how different levels of force may be applied duringsealing/deplating to establish electrical contact between the cleaningdisk and unwanted deposits. The cleaning disk 402 coated withcorrosion-resistant conductive layer 404 is positioned on top of the lipseal 406 and the electrical contacts 408 (only one electrical contact isshown in FIG. 4). The lip seal 406 is supported by the cup 410. Anunwanted metal deposit 444 is present on the lip seal, but it is removedfrom the wafer-lip seal interface by some distance. It is not unusualfor some deposits to remain unconnected after sealing. However, it isdesirable for the conductive layer 404 on the cleaning disk 402 to comeinto direct contact with the unwanted deposit 444, in order to providean electrical connection and help dissolve the deposit 444. In order toestablish this contact, a downward force is applied to the wafer(represented by the downward arrows). This force is transmitted throughthe wafer to the lip seal, which deforms in response to the appliedforce. This deformation can be seen in the middle and bottom panels ofFIG. 4. As different levels of force are used, the lip seal deforms andthe cleaning disk establishes electrical contact with unwanted depositclusters at various locations that may not have been previously incontact with the front side of the cleaning disk. When a relativelysubstantial force is applied, the cleaning disk 402 may press down uponand thereby move the electrical contacts 408. As such, the electricalcontacts should be flexible enough to withstand this movement.

Once the cleaning disk is sealed in the electroplating cup, the processmay proceed with submerging the exposed portion of the front surface ofthe cleaning disk into an electroplating solution. The electroplatingsolution may be the same as a solution used for plating regularsubstrates. In specific embodiments related to tin-silver plating, anelectroplating solution used for cleaning may include 0.2 to 2 g/l ofsilver ions, 30 to 120 g/l tin ions, and 70-350 or more g/l acid (basedon sulfuric acid or methane sulfonic acid). The solution may alsoinclude organic additives, such as grain refiners, noble metalcomplexers, brighteners, accelerators, suppressors, and/or levelers.Examples of suitable grain refiners include but are not limited to PEG,hydroxylated cellulose, gelatin, and peptone. Accelerators, suppressors,brighteners, and levelers are organic bath additives capable ofselectively enhancing or suppressing rates of deposition of metal ondifferent surfaces of the wafer features, thereby improving theuniformity of deposition. Complexing agents suitable for complexingsilver include aromatic thiols or sulfide compounds includingthiophenol, mercaptophenol, thiocresol, nitrothiophenol, thiosalicylicacid, aminothiophenol, benzenedithiophenol, mercaptopyridine.4,4-thiodiphenol, 4,4-aminodiphenyl sulfide, thiobisthiophenol,2,2-diaminodiphenyl disulfide, 2,2-dithiodibenzoic acid, ditolyldisulfide and 2,2-dipyridyl disulfide. These complexing agents may beused as silver complexers at low pH (e.g., pH less than about 2) and aresuitable for use in tin-silver plating baths (e.g., baths containingmethanesulfonic acid).

The cleaning method may proceed with applying a reverse current throughthe front surface of the cleaning disk to electrochemically dissolve thedeposits in the plating solution. During this reverse current stage, thecleaning disk serves as an anode (a positive potential is applied toit), and any unintended deposits at the lip seal that are electronicallyconnected with the disk also act as an anode.

The reverse current may be applied in either a potentiostatic mode or ina galvanostatic mode, in certain cases. In other cases, a combination ofthese two modes is used. In some embodiments, the current density iscontrolled to avoid oxygen generation as explained herein. The reversecurrent may be applied for a predetermined period of time or until athreshold is reached (e.g., until a certain current density is reachedin potentiostatic mode, or until a target potential is reached ingalvanostatic mode). The reference electrode may be used to control someof these process parameters. The time for applying the reverse currentmay depend on the rate and charge passed on product wafers. It may alsodepend on cleaning frequency. In certain embodiments, applying thereverse current is performed in two stages, each stage being performedat a different voltage potential relative to the other stage asexplained herein. The first voltage will generally be lower, andcorrespond to the metal in the alloy having a lower voltage potential(for example tin), while the second voltage will generally correspond tothe other metal (e.g., silver, the metal having a higher, more positivevoltage). In certain embodiments, applying the reverse current isperformed in a pulsed mode.

When appropriate anodic voltages are applied to the deplating disk andmetallic deposits, a corrosion potential can be established and acorrosion current can be passed directly (though contact between thedisk metal and unwanted lip seal metal) and indirectly (via an ionicreaction couple though the ionic solution) to the metallic deposits atthe substrate-to-lip seal interface. The metal deposits will startoxidizing and dissolving into electrolytic ions (Sn²⁺, Ag⁺, M⁺ whereM=metal), thereby removing the unintended deposits on the lip seal andcup bottom. The process may be performed either at a fixed appliedanodic potential (e.g. fixed vs. a reference electrode or vs. thecounter electrode) or using a constant current. In a particularembodiment, initially a constant current is applied, until reaching acritical switch-over potential, after which the process is performed ina potentiostatic mode. The applied currents, constant voltage value andthe potential for the switchover may be established after consideringand balancing the efficacy of removing the metals, and in inhibiting theprocess by creating significant amounts of gas (oxygen), which may blockthe process from occurring. In certain implementations of thisembodiment, the critical switch-over potential is effectively slightlybeyond the tin oxidation potential (as used herein, “slightly beyond”means above and within about 0.1-0.4 V of the cited potential, though insome cases the voltage may be more than 0.4 V beyond the citedpotential).

Without being restricted to any particular theory, it is believed that,in certain embodiments, a direct electrical connection between theconductive portion of the cleaning disk and unintended deposits on theelectroplating cup is not always needed, and indirect corrosion currentmay be sufficient to remove some, particularly smaller, deposits. Forexample, oxygen formation (with our without gas evolution) may occur atthe cleaning disk acting as an anode. At the same time, hydrogenevolution may occur at the counter electrode of the cell as well asmetal deposition. This process creates a voltage gradient in theelectrolyte, including the electrolyte adjacent to deposits that mayremain unconnected. At sufficiently high fields, portions of thedeposits that are farthest away from the cleaning disk will acquire anegative charge and plate tin as well, while deposits closest to thedisk will be positively charged and corrode metal(s) of these deposits.The net result will be movement of the deposits away from the disk andconsequentially away from the substrate-to-lip seal interface. Thepresence of unintended deposits at this interface is particularlyproblematic as they can easily make electrical connections to thesubstrate during later plating cycles. At the same time, deposits awayfrom this interface are less problematic and may remain relatively inertfor significant periods of time. Therefore, even if metals contained inthe unintended deposits are not actually removed from the electroplatingcup surfaces but are merely moved away from the substrate-to-lip sealinterface, the necessary cleaning is effectively achieved.

In certain embodiments, the electroplating cup is rotated within theelectroplating solution during application of the reverse current. Therotation speed of the electroplating cup may be at least about 5 RPM, atleast about 30 RPM, at least about 50 RPM, and in some cases is up toabout 180 RPM. In certain embodiments, the electroplating cup is rotatedin two opposite directions within the electroplating solution duringapplication of the reverse current (i.e., the cleaning disk may be firstrotated in one direction and then in an opposite direction).

After application of the reverse current is completed and the cleaningdisk is removed from the plating solution, the disk may be spun toreclaim the electrolyte off its surface before the seal is broken. Therotation speed during this operation may be between about 100 RPM and600 RPM. Sometimes this operation is referred to as spin drying. Thecleaning disk may be then rinsed with DI water while being rotated atsufficiently high speed to remove the remaining electrolyte on the wafersurface followed by another spin drying cycle. The cleaning disk is thenremoved from the electroplating cup and moved to the spin rinse drying(SRD) module of the plating apparatus described below. In the SRDmodule, the cleaning disk may be rinsed with water to remove anyremaining trapped electrolyte, followed by a high speed dry. Afterwards,the cleaning disk is extracted by the front end robot and returned dryto the dummy wafer storage area for reuse in another cleaning operation.With proper selection of materials and operating conditions during thecleaning process, the cleaning disk should not be subjected to removalor deposition of any materials on its surfaces. The cleaning disk may becontinuously reused.

In certain embodiments, the cleaning process may be performed inmultiple stages, e.g., a first stage configured to dissolve one metalfrom the alloy deposits and another stage configured to dissolve anothermetal of this alloy. For example, a voltage during the first stage maybe controlled such that this stage is completed at a voltage that isslightly beyond the tin oxidation potential as described above. Thisstage may be followed by another stage performed at a higher anodicvoltage to oxidize silver. In other embodiments, a process may beperformed in a galvanostatic regime, whereby the appropriate effectivecurrent density is between about 0.05 to 2 Amps per Square Decimeter.

A cleaning disk typically has a front surface having a corrosionresistant conductive material. This front surface is also referred to asthe deplating face. Some examples of the corrosion resistant materialsinclude noble corrosion resistant and conductive materials such asplatinum, palladium, gold, iridium, osmium, ruthenium, rhodium, andosmium, and chemically passivated materials such as titanium, tantalum,chromium, vanadium, tungsten, and niobium. The conductive material doesnot need to extend over the entire front surface of the cleaning disk.In certain embodiments, a center portion of the front side of thecleaning disk is covered by an insulating material, such as polyvinylchloride (PVC). In certain cases, the cleaning disk may include aninsulated center portion that is surrounded by a conductive layer, theconductive layer forming a ring around the insulated center portion andextending at least to an edge of the body. The cleaning disk itself mayhave the same diameter as a standard electroplating wafer. FIGS. 5A and5B illustrate front and side views, respectively, of cleaning disk 500in accordance with certain embodiments. The insulated portion 502 isshown with a white circle, while the conductive portion 504 is shownwith a grayed ring. In certain embodiments, the width of conductiveportion 504 is between about 1 millimeter and 10 millimeters or, morespecifically, between about 2 millimeters and 6 millimeters.

This width may define, at least in part, a gap between the insulatedcenter portion and lip seal after sealing the cleaning disk in theelectroplating cup. In certain embodiments, this gap is between about 1millimeter and 5 millimeters. The conductive portion 504 may be providedas a layer on otherwise insulated body 506. One example of the insulatedbody is a silicon wafer. In other embodiments, the entire body 506 isconductive and may itself form the conductive surface.

In certain embodiments, the front surface of the cleaning disk includesa conductive step extending perpendicular to the front surface. FIG. 6illustrates step 614 extending downward (in the direction opposite ofthe positive Z direction) from body 606. Step 614 may be formed byextending a conductive portion 604 over the insulated portion 602 asshown, or by specially conforming the shape of conductive portion 604.In certain embodiments, step 614 and conductive portion 604 are separatecomponents that may come in contact during sealing of the cleaning diskin the electroplating cup.

The height of the step (in the Z direction) may be at least about 1millimeter or, more specifically, at least about 2 millimeters. Having aconductive element extending along the Z axis allows making electricalconnections with additional deposits to be removed during the cleaningoperation. As shown in FIG. 6, deposits 608 may follow the shape of lipseal 612. A large portion of these deposits 608 may extend down onto thevertical surface (at least prior to deformation of lip seal 612 by thecleaning disk). Extending a conductive element in this direction allowsestablishing more connections between deposits 608 and this element.Step 614 may contact a portion of lip seal 612 after sealing thecleaning disk in the electroplating cup. In other embodiments, there maysome gap between step 614 and lip seal 612 even after sealing thecleaning disk as shown in FIG. 6. For example, the step may bepositioned within 2 millimeters of the lip seal after sealing thecleaning disk. Even with a gap, step 614 may provide sufficient currentdensity within the electrolyte in order to electrochemically dissolvedeposits on the other side of this gap due to polarization of theelectrolyte as explained above. In certain embodiments, the lip sealdeforms and extends towards the step when the cleaning disk is sealed inthe electroplating cup.

Even if after performing a cleaning process, some deposits remain on theelectroplating cup, these deposits remain away from the substrate-to-lipseal interface (e.g., at least 2 mm away from this interface, and insome cases at least 5 mm or 10 mm away from this interface).Furthermore, substantial amounts of initial deposits are expected to beactually removed (e.g., at least 25% of the deposits are removed, forexample at least 50% of the deposits are removed, by mass). Overall,disruptive characteristics of initial unintended deposits will besignificantly diminished and plating operations may be continued withoutrisk of catastrophic failures.

This cleaning process may be applied after pre-determined periodicsubstrate-processing intervals, analogous to maintenance cycles. Theprocess may repeat after a certain number of wafers are processed, aftera certain amount of time has passed, after a certain amount of chargehas passed, etc. In some cases, where a detection method is used todetect the presence of plated metal on the lip seal or other part of thewafer holder assembly, the detector may send a signal that cleaning isappropriate. In these cases, cleaning may be repeated whenever such a“cleaning required” signal is received. The frequency of cleaningdepends on many factors, such as wafer open area, bath composition, andtemperature as well as other operating conditions. Preventivemaintenance that involves physical wiping of the electroplating cupsurfaces with acidic solution can be performed less frequently whencombined with the proposed cleaning processes. Furthermore, the proposedcleaning processes can be implemented with very few modifications toexisting plating apparatuses and without introducing any modificationsto plating solutions or risk of contamination to the plating solution.

Processes for cleaning wafer plating assemblies are further described inU.S. patent application Ser. No. 13/563,619, titled “AUTOMATED CLEANINGOF WAFER PLATING ASSEMBLY,” filed on Jul. 31, 2012, which isincorporated herein by reference in its entirety.

Apparatus

The methods described herein may be performed by any suitable apparatus.A suitable apparatus includes hardware for accomplishing the processoperations and a system controller having instructions for controllingprocess operations in accordance with the present invention. In someembodiments, the hardware may include one or more process stationsincluded in a process tool.

FIG. 7 provides details on the cup and cone assembly of the clamshellaccording to an embodiment herein. The figures is cross-sectionalschematic representation of a portion 701 of assembly 700 including cone703 and cup 702. This figure is not meant to be an accurate depiction ofthe cup and cone assembly, but rather a stylized depiction fordiscussion purposes. Cup 702 is supported by top plate 705 using struts704. Generally, cup 702 provides a support upon which substrate 745rests. Cup 702 includes an opening through which electrolyte from aplating cell can contact substrate 745 provided at the opening. Notethat substrate 745 has a front/working side 742, which is where platingoccurs. The periphery of substrate 745 rests on a bottom inwardprotrusion of cup 702 (e.g., “knife-shaped” edge) or more specificallyon lip seal 743 positioned on the bottom inward protrusion of cup 702.Cone 703 presses down on the back side of substrate 745 to hold it inplace and to seal it with lip seal 743 during submersion of theelectroplating cup into the plating solution and during actual plating.

During the cleaning process describe herein, substrate 745 is replacedwith a cleaning disk, which is positioned and sealed with theelectroplating cup in a similar manner. As such, any discussion ofsubstrate 745 in relation to FIG. 7 applies equally well to the cleaningdisk when a cleaning method is being performed (with the exception ofthe current applied, which is reversed during the cleaning process ascompared to a conventional plating process). The cleaning disk has aportion of its front surface made from a corrosion resistant conductivematerial. This conductive portion contacts unintended deposits on thecup as the cleaning disk is sealed in the cup. The electroplating cupcontaining and sealing the cleaning disk is then submerged into theelectrolyte in a manner similar to the regular plating process. However,instead of applying a direct current to the conductive portion of thecleaning disk which would have caused deposition of metals on thissurface, the reverse current is applied causing cleaning and deplatingof the unintended residues from the electroplating cup surfaces.

Returning to the FIG. 7, loading of wafer 745 into 701 is performed bylifting cone 703 from its depicted position via spindle 706. When cone703 is lifted, a gap is created between cup 702 and cone 703 into whichsubstrate 745 can be inserted. Cone 703 is then lowered to engagesubstrate 745 against the periphery of cup 702 as depicted or, morespecifically, against lip seal 743 supported by a bottom inwardprotrusion of cup 702.

Spindle 706 may be used to transmit a vertical force for engaging cone703 to substrate 745 and to rotate the entire assembly 701 duringelectroplating, drying, and other operations. These transmitted forcesare indicated by the up/down arrow in FIG. 7 and the downwards arrows inFIG. 4. Note that substrate plating typically occurs while the substrateis being rotated with the plating solution. As explained above, assembly701 may include a compressible lip seal 743 to form a fluid-tight sealwhen cone 703 engages substrate 745. The vertical force from cone 703,which is transferred through substrate 745 compresses lip seal 743 toform the fluid tight seal. Lip seal 743 prevents electrolyte fromcontacting the backside of wafer 745 (where the electrolyte couldintroduce contaminating metal atoms directly onto/into the siliconsubstrate) and from reaching sensitive components of apparatus 701, suchas contact fingers that establish electrical connections to the edgeportions of substrate 745. This electrical connection is used to supplycurrent to conductive portions of substrate 745 that are exposed to theelectrolyte. Overall, lip seal 743 separates unexposed edge portions ofsubstrate 745 from exposed portions of substrate 745. Both portionsinclude conductive surfaces that are in electrical communication witheach other.

Cone 703 also includes seal 749. As shown, cone seal 749 is located nearthe edge of cone 703 and an upper region of the cup when engaged. Thisarrangement also protects the backside of wafer 745 from any electrolytethat might enter the clamshell from above the cup. Seal 749 may beaffixed to the cone or the cup, and may be a single seal or amulti-component seal. Upon initiation of plating, wafer 745 isintroduced to assembly 702 when cone 703 is raised above cup 702. Whenthe wafer is initially introduced into cup 702, typically by a robotarm, its front side 742 rests lightly on lip seal 743. During plating,the assembly 701 rotates in order to aid in achieving uniform plating.

FIG. 8 provides a perspective view of a wafer holding and positioningapparatus 800 for electrochemically treating semiconductor wafers.Apparatus 800 includes wafer engaging components, which are sometimesreferred to herein as “clamshell” components. The actual clamshellincludes cup 802 and cone 803 that clamps a wafer securely in cup 802.Cup 802 is supported by struts 804, which are connected to top plate805. This assembly (802-805), collectively assembly 801, is driven by amotor 807, via a spindle 806. Motor 807 is attached to a mountingbracket 809. Spindle 806 transmits torque to a wafer (not shown in thisfigure) to allow rotation during plating. An air cylinder (not shown)within spindle 806 also provides vertical force to clamp the waferbetween the cup and cone 803. For the purposes of this discussion, theassembly including components 802-809 is collectively referred to as awafer holder 811. Note however, that the concept of a “wafer holder”extends generally to various combinations and sub-combinations ofcomponents that engage a wafer and allow its movement and positioning.

A tilting assembly, which includes first plate 815 slidably connected tosecond plate 817, is connected to mounting bracket 809. Drive cylinder813 is connected both to plate 815 and plate 817 at pivot joints 819 and821, respectively. Thus, drive cylinder 813 provides force for slidingplate 815 (and thus wafer holder 811) across plate 817. The distal endof wafer holder 811 (i.e., mounting bracket 809) is moved along an arcedpath (not shown) which defines the contact region between plates 815 and817, and thus the proximal end of wafer holder 811 (i.e., cup and coneassembly) is tilted upon a virtual pivot. This allows for angled entryof a wafer into a plating bath.

The entire apparatus 800 is lifted vertically either up or down toimmerse the proximal end of wafer holder 811 into a plating solution viaanother actuator (not shown). Thus, a two component positioningmechanism provides both vertical movement along a trajectoryperpendicular to an electrolyte and a tilting movement allowingdeviation from a horizontal orientation (parallel to electrolytesurface) for the wafer (angled-wafer immersion capability).

Note that apparatus 800 is typically used with a particular plating cellhaving a plating chamber which houses an anode and electrolyte. Theplating cell may also include plumbing or plumbing connections forcirculating electrolyte through the plating cell—and against the workpiece being plated. It may also include membranes or other separatorsdesigned to maintain different electrolyte chemistries in an anodecompartment and a cathode compartment.

Reference is now made to FIG. 9, which is a schematic drawing of onesuitable automated wafer plating tool apparatus 900 configured to platea semiconductor wafer (e.g., silicon) or similar substrate (e.g.,glass-coated thinned wafer, GaAs, ceramic, etc.) with various metals andalloys (e.g. copper, nickel, gold, palladium, cobalt, indium, tin, lead,lead-tin, tin-silver, FeCo, etc.), and perform other necessary platingsub-processes (e.g. spin rinsing and drying, metal and silicon wetetching, electroless deposition, pre-wetting and pre-chemical treating,photoresist stripping, surface pre-activation, etc.). The tool is shownschematically looking top down, and shows only a single level or“floor”, but it is understood that the tool, such as the Novellus Sabre™3D tool, can have two or more levels “stacked” on top of each otherincluding identical or different types of processing stations.

As mentioned, of particular interest are scenarios including plating oftin silver solder alloys, which are known to generally have the problemof creating spurious metal deposits on and around the wafer holding“cup” and lip-seal over time and with use, and which therefore requireperiodic maintenance to remove and clean the metallic and other filmbuildup. The spurious plated metal on the sealing area and its vicinitydivert and otherwise change the desired pattern of current distribution,and its removal is therefore required to maintain good within waferuniformity and particle performance.

Substrates 906 that are to be processed are generally fed to the toolthrough front end loading “FOUP” 901, and in this example are broughtfrom the FOUP 901 to the main substrate processing area of the tool viafront end robot 902. Front end robot 902 can retract and move substratesin multiple dimensions from one station to another of the accessiblestations, such as two front end accessible module pre-treating stations904 and cleaning stations 908 as shown in this example. Lateral movementfrom side to side of the robot carrying the wafer is accomplished byrobot track 902A.

After determining that a number of processed substrates 906 in thetin-silver plating cell 907 reaches a predetermined level (e.g., 50 to200 wafers or more, depending on the deposition rate and charge passedper wafer), the system 900 initiates a cleaning process. Where adetection assembly is used to detect the presence of unwanted deposits,the system 900 may initiate a cleaning process in response to a signalfrom the detector that cleaning is appropriate. The two plating cells907 are then denoted as not available for further plating until thecleaning is completed. In a manual operation procedure the cells wouldneed to be denoted as having completed the manual cleaning operation bythe operator, but in the automated process, the system softwareinitiates the automatic cleaning sequence and, when complete, allows forfurther wafer processing through those cells thereafter until thepre-designated cleaning interval or other criteria is met again.

The system may instruct substrate handling robots to extract cleaningdisk from the “dummy-wafer” storage area 903, also referred to as acleaning disk storage holder. Various examples of cleaning disks aredescribed above. Cleaning disk 920 may be retrieved by front end robot902, handed over to backend robot 909, and transported by backend robot909 to tin-silver plating station 907. Backend robot 909 may also beused to insert cleaning disk 920 into the electroplating cup/clamshellof plating station 907 as described above and in a manner similar toinserting regular substrates for plating. As with the front end robot902, the backend robot 909 is also shown having a track and is capableof transporting the substrate 906 or the cleaning disk 920 from theforward pre-treating stations and from either the upper or lower decksof the tool to any other tool station throughout the tool.

Once the backend robot 909 places the cleaning disk into theelectroplating cup, the wafer holding cup-clamshell is closed and a sealis made between the edge of the disk and the lip seal as describedabove. With the cleaning disk in place, the electroplating cup issubmerged into the plating cell electrolyte, and the process ofdeplating is ready to be initiated. The process is initiated by passingan anodic current on the cup side in either a potentiostatic orgalvanostatic mode as described above. Suitable rotation speeds rangefrom about 0 to 180 RPM.

Upon completion of the cleaning operations, the cleaning disk 920 isremoved from the plating solution, and is optionally spun at a speed ofbetween about 100 to 600 RPM to reclaim the electrolyte off its surface.This is followed an optional DI rinsing operating of the deplating diskat sufficiently high speeds to rinse off remaining electrolyte residueon the wafer surface. The cleaning disc 920 is then cleaned and storedin the storage area 903.

An electroplating apparatus suitable for use with the embodiments hereinis further described in U.S. patent application Ser. No. 13/305,384,titled “ELECTROPLATING APPARATUS AND PROCESS FOR WAFER LEVEL PACKAGING,”filed on Nov. 28, 2011, which is incorporated by reference herein in itsentirety.

The electroplating system may include a system controller for receivingfeedback signals from various modules of the system and supplyingcontrol signals to the same or other modules. The system controller maycontrol operation of electroplating cups, robots, application ofcurrents, and many other process variables. In certain embodiments, thesystem controller may synchronize the operation of the electroplatingcups and robots with respect to other modules. In more specificembodiments, the system controller may determine when a cleaning processshould be initiated in a sequence of plating operations (according to,e.g., a periodic cleaning schedule based on the number of wafers plated,the amount of time passed, the amount of charge passed, or a signal froma detector that cleaning is required).

In the depicted embodiment, the system controller is employed to controlprocess conditions during various operations described above. Someexamples of such operations include providing a cleaning disk, sealingthe cleaning disk in an electroplating cup, applying a force to thecleaning disk, changing the force applied to the cleaning disk,submerging the electroplating cup into the plating solution, applyingreverse current, and ceasing the application of current when thecleaning process is complete.

The system controller will typically include one or more memory devicesand one or more processors. The processor may include a centralprocessing unit (CPU) or computer, analog and/or digital input/outputconnections, stepper motor controller boards, and other like components.Instructions for implementing appropriate control operations areexecuted on the processor. These instructions may be stored on thememory devices associated with the controller or they may be providedover a network.

In certain embodiments, the system controller controls all or mostactivities of the semiconductor processing system described above. Forexample, the system controller may control all or most activities of thesemiconductor processing system associated with cleaning anelectroplating cup from unintended deposits caused by plating alloymaterials. The system controller executes system control softwareincluding sets of instructions for controlling the timing of theprocessing steps, pressure levels, flow rates, and other parameters ofparticular operations further described below. Other computer programs,scripts, or routines stored on memory devices associated with thecontroller may be employed in some embodiments.

Typically, there is a user interface associated with the systemcontroller. The user interface may include a display screen, graphicalsoftware to display process conditions, and user input devices such aspointing devices, keyboards, touch screens, microphones, and other likecomponents.

The computer program code for controlling the above operations can bewritten in any conventional computer readable programming language: forexample, assembly language, C, C++, Pascal, Fortran or others. Compiledobject code or script is executed by the processor to perform the tasksidentified in the program.

Signals for monitoring the process may be provided by analog and/ordigital input connections of the system controller. The signals forcontrolling the process are output on the analog and digital outputconnections of the processing system.

The cleaning process of the plating apparatus and lip seal using acleaning disk are described above with reference to an automated tooland integrated processing approach. It is understood that those skilledin the art would appreciate that other approaches, including certainmanual operations, can be substituted to accomplish some of theoperations described.

The various hardware and method embodiments described above may be usedin conjunction with lithographic patterning tools or processes, forexample, for the fabrication or manufacture of semiconductor devices,displays, LEDs, photovoltaic panels and the like. Typically, though notnecessarily, such tools/processes will be used or conducted together ina common fabrication facility.

Lithographic patterning of a film typically comprises some or all of thefollowing steps, each step enabled with a number of possible tools: (1)application of photoresist on a workpiece, e.g., a substrate having asilicon nitride film formed thereon, using a spin-on or spray-on tool;(2) curing of photoresist using a hot plate or furnace or other suitablecuring tool; (3) exposing the photoresist to visible or UV or x-raylight with a tool such as a wafer stepper; (4) developing the resist soas to selectively remove resist and thereby pattern it using a tool suchas a wet bench or a spray developer; (5) transferring the resist patterninto an underlying film or workpiece by using a dry or plasma-assistedetching tool; and (6) removing the resist using a tool such as an RF ormicrowave plasma resist stripper. In some embodiments, an ashable hardmask layer (such as an amorphous carbon layer) and another suitable hardmask (such as an antireflective layer) may be deposited prior toapplying the photoresist.

It is to be understood that the configurations and/or approachesdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered in a limiting sense,because numerous variations are possible. The specific routines ormethods described herein may represent one or more of any number ofprocessing strategies. As such, various acts illustrated may beperformed in the sequence illustrated, in other sequences, in parallel,or in some cases omitted. Likewise, the order of the above describedprocesses may be changed.

The subject matter of the present disclosure includes all novel andnonobvious combinations and sub-combinations of the various processes,systems and configurations, and other features, functions, acts, and/orproperties disclosed herein, as well as any and all equivalents thereof.

1. A method of mitigating an electroplating deposit on a substrateholder that is configured to hold a wafer substrate in an electroplatingcell while applying a cathodic current to electroplate metal on thesubstrate, the method comprising: when the electroplating cell is notbeing used for electroplating metal on a substrate, providing adeplating disk in the substrate holder such that the deplating diskmakes electrical contact with a plurality of electrical contacts in thesubstrate holder; immersing the deplating disk and substrate holder intoan electroplating solution within the electroplating cell; and applyingan anodic potential to the deplating disk under conditions sufficient toremove, at least partially, the electroplating deposit from thesubstrate holder, wherein the deplating disk has a size and a shapematching those of a standard semiconductor wafer.
 2. The method of claim1, wherein the deplating disk comprises a metal around its perimeter ina region for making said electrical contact with said plurality ofelectrical contacts in the substrate holder.
 3. The method of claim 2,wherein the metal is a noble metal.
 4. The method of claim 2, whereinthe deplating disk comprises an insulator in a central region of thedeplating disk.
 5. The method of claim 1, wherein the electroplatingdeposit on a substrate holder comprises silver and tin.
 6. The method ofclaim 1, wherein the substrate holder comprises a lip seal.
 7. Themethod of claim 6, further comprising compressing the deplating disk onthe lip seal to cause the deplating disk to come into electrical contactwith the electroplating deposit.
 8. The method of claim 7, furthercomprising: after applying the anodic potential to the deplating disk,varying compression on the deplating disk to cause the lip seal to flexto a new position and thereby cause the deplating disk to come intoelectrical contact with the electroplating deposit;
 9. The method ofclaim 1, further comprising moving the deplating disk to a storage areafor the deplating disk.
 10. A method of cleaning a semiconductor platingapparatus, comprising: providing a deplating disk in a wafer holderassembly, wherein the deplating disk comprises corrosion-resistantconductive material around at least the periphery of the deplating disk,said wafer holder assembly comprising a peripheral lip seal and aplurality of electrical contacts, wherein the wafer holder assembly ispositioned in an electroplating cell, and wherein the wafer holderassembly has metal deposited thereon, and sealing the deplating disk inthe wafer holder assembly by applying a force to the deplating disk tothereby deform the lip seal and provide a liquid-tight seal, and therebyestablish electrical communication between the plurality of electricalcontacts and the metal deposited on the wafer holder assembly,submerging the deplating disk in cleaning solution in the electroplatingcell, and applying anodic current to the deplating disk to therebyremove metal deposited on the wafer holder assembly.
 11. The method ofclaim 10, further comprising changing the force applied to the deplatingdisk as current is applied to further deform the lip seal and therebyprovide new electrical connections between the deplating disk and metaldeposited on the wafer holder assembly.
 12. The method of claim 10,wherein the force applied to the deplating disk is the same as a forceapplied to a plating wafer when a plating wafer is sealed in the waferholder assembly.
 13. The method of claim 10, wherein the force appliedto the deplating disk is different from a force applied to a platingwafer when a plating wafer is sealed in the wafer holder assembly. 14.The method of claim 10, wherein the deplating disk comprises aninsulated center portion.
 15. The method of claim 10, wherein the metaldeposited on the wafer holder assembly comprises an alloy.
 16. Themethod of claim 15, wherein the alloy comprises tin and silver.
 17. Themethod of claim 15, wherein applying anodic current to the deplatingdisk comprises: applying current at a first voltage potential, andapplying current at a second voltage potential, wherein the firstpotential corresponds to a potential that is slightly beyond the voltagepotential of a metal in the alloy.
 18. The method of claim 10, whereinthe cleaning solution is the same as a solution used for electroplatinga plating wafer.
 19. The method of claim 10, wherein when applyinganodic current to the deplating disk, the current density is controlledto avoid oxygen generation.
 20. The method of claim 18, wherein anodiccurrent is applied to the deplating disk in a potentiostatic mode. 21.The method of claim 20, wherein anodic current is applied to thedeplating disk until a target current density is reached.
 22. The methodof claim 10, wherein anodic current is applied to the deplating disk ina galvanostatic mode.
 23. The method of claim 22, wherein a currentdensity is between about 0.05-2 Amps per square decimeter.
 24. Themethod of claim 10, wherein anodic current is applied to the deplatingdisk in pulses.
 25. The method of claim 10, further comprising rotatingthe deplating disk at about 30 RPM or greater during at least theapplying current operation.
 26. The method of claim 10, wherein thedeplating disk is rotated in a first direction and then in a seconddirection.
 27. The method of claim 10, further comprising: removing thedeplating disk from the cleaning solution; spinning the deplating diskbetween about 100-600 RPM to reclaim the cleaning solution from thedeplating disk; rinsing the deplating disk with deionized water; andspinning the deplating disk to remove the deionized water.
 28. Themethod of claim 10, further comprising repeating the methodperiodically.
 29. The method of claim 28, wherein the method is repeatedafter a time interval.
 30. A plating tool for plating semiconductorwafers, comprising, at least one electroplating cell configured tocontain a metal-ion-containing electroplating solution; a plating cellwafer holder assembly comprising a peripheral lip seal and a pluralityof electrical contacts, said plating cell wafer holder assemblyconfigured to receive a workpiece comprising either a semiconductorwafer or a deplating disk, wherein said peripheral lip seal isconfigured to form a liquid-tight seal between an edge of the workpieceand the peripheral lip seal to thereby prevent electroplating solutionfrom coming into contact with the plurality of electrical contacts; apower supply configured to provide anodic current to an anode andcathodic current to the workpiece in a plating mode, and anodic currentto the workpiece in a cleaning mode; a deplating-disk storage holder; awafer storage area; a robotic arm; and a controller configured to directthe robotic arm to move the deplating disk between the deplating diskstorage holder and the electroplating cell, and to move thesemiconductor wafer between the wafer storage area and theelectroplating cell, and to direct the power supply to supply currentaccording to the plating mode when the semiconductor wafer is present inthe electroplating cell, and to supply current according to the cleaningmode when the deplating disk is present in the electroplating cell.